In German Offenlegungsschrift No. 2,504,081 (British Pat. No. 1,495,598), there has been described a process for the tanning or re-tanning of pelts or leather, which is characterized in that pelts or leather are treated with an aqueous liquor containing:
(a) hydrophilic oligourethanes which may be methylolated and which have a molecular weight (Mn) of from 300 to 20,000; and, optionally,
(b) formaldehyde or a substance from which formaldehyde is split off.
The oligourethanes described there are generally based on water-soluble polyethers having a molecular weight below 500 and they are preferably free from ionic groups. However, German Offenlegungsschrift No. 2,504,081 (British Pat. No. 1,495,598) also mentions oligourethanes which contain sulphonate groups and are based on hydrophobic, water-insoluble polyethers. It is expressly pointed out in this connection that the incorporation of ionic groups into the oligourethane resin should be considered in cases where hydrophobic polyhydroxyl compounds or combinations of hydrophobic with non-ionic, hydrophilic polyhydroxyl compounds are used for the synthesis of the oligourethane. As is well known to those skilled in the art, the incorporation of ionic centers enables even those oligourethanes to be dissolved in water which, in the absence of the ionic center, are neither soluble in water nor dispersable therein.
In the course of further development of the known process, it has now surprisingly been found that aqueous solutions of such oligourethane resins are greatly improved in the tanning and dyeing characteristics thereof, if, in addition to hydrophilic polyethylene glycol polyethers, they contain cationic centers obtained in known manner by the reaction of a tertiary nitrogen atom with a quaternizing agent and/or an acid.
It is found particularly advantageous to use the quaternizing agent and/or the acid in sub-equivalent quantities, based on the tertiary nitrogen, so that the pH of the resulting aqueous solution of the cationic oligourethane is between about 5 and 10, preferably between about 7 and 9.
The aqueous oligourethane solutions according to the present invention have the following advantages over solutions known in the art:
1. They have excellent resistance to electrolytes in spite of the cationic character thereof.
2. The anionic character of re-tanned chrome leather is over-compensated.
3. The products have a soft tanning effect.
4. The products may be used for pre-tanning without deleteriously affecting the subsequent chrome tanning.
5. The cationic oligourethane solutions improve the consumption of the anionic dyes and produce a more brilliant and more intense color without deleteriously affecting the levelling of the dyeing.